Automatic detection and tracking of multiple individuals using multiple cues

ABSTRACT

Automatic detection and tracking of multiple individuals includes receiving a frame of video and/or audio content and identifying a candidate area for a new face region in the frame. One or more hierarchical verification levels are used to verify whether a human face is in the candidate area, and an indication made that the candidate area includes a face if the one or more hierarchical verification levels verify that a human face is in the candidate area. A plurality of audio and/or video cues are used to track each verified face in the video content from frame to frame.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/006,927, filed Dec. 3, 2001, entitled “Automatic Detection andTracking of Multiple Individuals Using Multiple Cues”, which is herebyincorporated by reference herein.

TECHNICAL FIELD

This invention relates to image and/or audio processing, and/or computervision, and particularly to automatic detection and tracking of multipleindividuals.

BACKGROUND

Systems that analyze video data are becoming increasingly popular. Videoconferencing systems are examples of such systems—they allow formeetings to occur with visual interaction despite the fact that meetingparticipants may be located in different geographic locations. Thevisual aspect of video conferencing makes it typically more appealingthan telephone conferences, while at the same time being a lower-costalternative to (and typically can occur on shorter notice than)in-person meetings when one or more participants must travel to themeeting location.

Some current video conferencing systems use automated audio-baseddetection techniques and/or presets to move the camera (e.g., pan ortilt the camera). However, many problems exist with current videoconferencing systems. One such problem is that the accuracy ofaudio-based speaker detection technique can be low. Additionally, thevideo conferencing system typically does not know how many participantsthere are in the meeting (including when participants join or leave themeeting), where the participants are located (sitting or standing), orwhich participant is currently talking. While some systems may bemanually programmed with participant information (e.g., the number ofparticipants and their locations), this requires user-entry of theinformation being programmed, which tends to restrict participants'ability to move about the room, as well as the ability of participantsto join the conference.

The automatic detection and tracking of multiple individuals describedherein helps solve these and other problems.

SUMMARY

Automatic detection and tracking of multiple individuals is describedherein.

According to one aspect, a frame of content (e.g., audio and/or video)is received and one or more candidate areas for a new face region in theframe are identified. Hierarchical verification is then used to verifywhether a human face is in the candidate area(s), and an indication madethat the candidate area(s) includes a face if the hierarchicalverification verifies that a human face is in the candidate area(s).After verification of the area(s), a plurality of cues are used to trackeach verified face in the content from frame to frame.

According to one aspect, there are three main modules in this detectionand tracking framework: an automatic initialization module, ahierarchical verification module, and a multi-cue tracking module. Aframe of content (e.g., audio and/or video) is received and one or morecandidate areas for a new face (or other object) region in the frame areidentified by the automatic initialization module. The hierarchicalverification module is then used to verify whether a human face is inthe candidate area(s), and an indication made that the candidate areaincludes a face if the hierarchical verification module verifies that ahuman face is in the candidate area(s). After the area(s) beingverified, the multi-cue tracking module uses a plurality of cues totrack each verified face in the content from frame to frame. During thewhole tracking process, the tracked faces are continuously verified bythe hierarchical verification module. If the confidence level is high,the multi-cue tracking module keeps track of the faces; if theconfidence becomes low, tracking of that particular face is terminated.The tracking module and verification module wait for the initializationmodule to supply more candidates.

BRIEF DESCRIPTION OF THE DRAWINGS

The same numbers are used throughout the document to reference likecomponents and/or features.

FIG. 1 illustrates an exemplary environment in which robust automatedidentification and tracking can be used.

FIG. 2 illustrates another exemplary environment in which robustautomated identification and tracking can be used.

FIG. 3 illustrates an exemplary system using robust automatedidentification and tracking.

FIG. 4 is a flowchart illustrating an exemplary process for detectingcandidates for new face regions.

FIG. 5 is a flowchart illustrating an exemplary process for identifyingcandidates for new face regions using motion-based initialization.

FIG. 6 illustrates an exemplary image for a frame of video content.

FIG. 7 is a flowchart illustrating an exemplary process for performinghierarchical verification.

FIG. 8 illustrates an exemplary process for fast color-basedverification.

FIG. 9 is a flowchart illustrating an exemplary process for performingmulti-cue tracking.

FIG. 10 illustrates exemplary modeling and comparing for multi-cuetracking in additional detail.

FIG. 11 is an image illustrating the region smoothness concept.

FIG. 12 illustrates measurements of intensities from FIG. 11.

FIG. 13 illustrates exemplary calculation of a matching distancegraphically.

FIG. 14 illustrates exemplary tracking of an object from one frame tothe next.

FIG. 15 is a flowchart illustrating an exemplary unscented particlefilter process.

FIG. 16 illustrates an exemplary multiple-microphone environment.

FIG. 17 illustrates an exemplary general computer environment.

DETAILED DESCRIPTION

Automatic detection and tracking of multiple individuals is describedherein. Video content and/or audio content is analyzed to automaticallydetect individuals in the frames of the content. Once detected, theseindividuals are automatically tracked in successive frames. In the eventthat tracking of an individual is lost, the individual is automaticallydetected again and tracking of the individual resumes.

FIGS. 1 and 2 illustrate exemplary environments in which robustautomated detection and tracking can be used. In FIG. 1, multiple (n)video conferencing systems 102 are able to communicate audio/videocontent to one or more of each other, allowing conference participantslocated at each of the systems 102 to see and hear one another. A widevariety of different camera systems may be used with video conferencingsystems 102, such as conventional pan/tilt/zoom cameras, 360-degreepanorama cameras (e.g., which can pan/tilt/zoom digitally rather thanmechanically), etc. One such 360-degree panorama camera system uses acamera pointed at a parabolic mirror device, and then uses variouscalibration techniques to de-warp the image to normal images from whicha 360-degree omni-directional image about the camera can be constructed.An example of such a 360-degree panorama camera system can be found inco-pending U.S. patent application Ser. No. 09/681,843, entitled“Automated Online Broadcasting System and Method Using anOmni-Directional Camera System for Viewing Meetings Over a ComputerNetwork”, filed Jun. 14, 2001, by inventors Yong Rui, Anoop Gupta,Johnathan J. Cadiz, and Ross G. Cutler. Another such 360-degree panoramacamera system uses multiple cameras (each having a less-than-360-degreefield of view) arranged so that together they provide an approximately360-degree field-of-view.

Each of conferencing systems 102 includes a tracking module 104 thatrobustly automatically detects and tracks multiple individuals at thecorresponding system 102. This detection and tracking can be used for avariety of purposes, such as to tilt/pan/zoom the camera, highlight anindividual (e.g., with an arrow pointing at or circle around theindividual), etc.

Video conferencing systems 102 can be coupled together in any of a widevariety of manners. For example, one or more telephone lines (includingdigital lines, such as ISDN) may be used to couple together multipleones of systems 102, either directly or via a central device orlocation, a conventional data network (e.g., the Internet, an intranet,etc.) may be used to couple together multiple ones of systems 102, andso forth.

In FIG. 2, a system 112 including a tracking module 114 receives content116. Content 116 is typically audio/video content, but alternatively mayinclude other types of content (e.g., shared whiteboard, etc.) and/ormay not include audio content or video content. Tracking module 114analyzes content 116 and robustly automatically detects and tracksmultiple individuals based on their images and/or audio in content 116.Content 116 can be made available to system 112 in any of a variety ofmanners, such as a camera and microphone at system 112, a recordingmedium (e.g., magnetic tape, optical disk, etc.) on which the content isrecorded, a telephone line or network input, etc.

FIG. 3 illustrates an exemplary system 130 using robust automateddetection and tracking. System 130 may be, for example, any of videoconferencing systems 102 of FIG. 1 or a system 112 of FIG. 2. System 130includes a detection and tracking module 132, a communications module134, an audio capture module 136, and a video capture module 138.Various other modules (not shown) may also be included, such as awhiteboard capture module. Communications module 134 managescommunications for system 130 with other systems, such as other videoconferencing systems 102 of FIG. 1, or other devices from which contentto be analyzed may be received. Communications module 134 can support awide variety of conventional and/or proprietary protocols.

Audio capture module 136 manages the capturing of audio content atsystem 130, such as via one or more microphones (not shown) that arepart of system 130. Further processing (e.g., using beamformingtechniques) can also be done to enhance the audio quality. The audiocontent is converted to digital format (if necessary) and is madeavailable to detection and tracking module 132 for tracking. Videocapture module 138 manages the capturing of video content at system 130,such as via one or more video capture devices (e.g., analog or digitalvideo cameras (not shown)) that are part of system 130 (which mayinclude, for example, fixed cameras, conventional pan/tilt/zoom cameras,360-degree panorama cameras, etc.). The captured frames of video contentare then converted to digital format (if necessary) and are madeavailable to detection and tracking module 132 for detection andtracking of individuals. The audio and video content are correlated withone another (e.g., at the time of capture), so for any particularportion of content (e.g., a frame) both the video and audio content areknown. In alternate embodiments, one or more of modules 134, 136, and138 may not be included. For example, a system may not include either avideo capture module 138 or an audio capture module 136.

Detection and tracking module 132 includes an auto-initialization module140, a hierarchical verification module 142, a multi-cue tracking module144, and a face/candidate tracking list 146. Detection and trackingmodule 132 automatically detects regions of video content that include,or potentially include, human faces, and uses various cues to track thedetected regions. These regions are also referred to herein as objects.Detection and tracking module 132 is capable of detecting multipleregions that include faces or face candidates and tracking thesemultiple regions concurrently.

Detection and tracking module 132 analyzes portions of content, such asframes. For example, video content is typically captured as a number offrames (e.g., still images) per second (typically on the order of 15-60frames per second, although other rates may be used). These videoframes, as well as the corresponding audio content (e.g., every 1/15 to1/60 of a second of audio data) are used as the frame for detection andtracking by module 132. When recording audio, the audio is typicallysampled at a much higher rate than the video (e.g., while 15 to 60images may be captured each second for video, thousands of audio samplesmay be captured). The audio samples may correspond to a particular videoframe in a variety of different manners. For example, the audio samplesranging from when a video frame is captured to when the next video frameis captured may be the audio frame corresponding to that video frame. Byway of another example, the audio samples centered about the time of thevideo capture frame may be the audio frame corresponding to that videoframe (e.g., if video is captured at 30 frames per second, the audioframe may range from 1/60 of a second before the video frame is capturedto 1/60 of a second after the video frame is captured).

Additionally, in some situations there may be no video content. In thesesituations, frames of audio content can be generated from the sampledaudio in any of a wide variety of manners. For example, the audiosamples for every 1/30 of a second or every 1/60 of a second mayconstitute the frame of audio content.

In some situations the audio content may include data that does notdirectly correspond to the video content. For example, the audio contentmay be a soundtrack of music rather than the voices of people in thevideo content. In these situations, the detection and tracking describedherein relies on the video content without the audio content.

Although discussed herein primarily with reference to using video andaudio content, detection and tracking module 132 may alternativelyoperate based on only video content or only audio content. In situationswhere there is no audio content, the processes discussed below forprocessing audio content are not performed. Similarly, in situationswhere there is no video content, the processes discussed below forprocessing video content are not performed.

Face/candidate tracking list 146 maintains information for each detectedregion that includes, or potentially includes, a human face. Thoseregions that potentially include a face but for which the presence of aface has not been verified are referred to as candidate regions. In theillustrated example, each region is described by a center coordinate148, a bounding box 150, a tracking duration 152, and a time since lastverification 154. The regions of video content that include faces orface candidates are defined by a center coordinate and a bounding box.Center coordinate 148 represents the approximate center of the region,while bounding box 150 represents a rectangular region around the centercoordinate. This rectangular region is the region that includes a faceor face candidate and is tracked by detection and tracking module 132.Tracking duration 152 represents how long the face or face candidate inthe region has been tracked, while the time since last verification 154represents how long ago the face or face candidate in the region wasverified (by verification module 142, as discussed in more detailbelow).

The information describing each region as illustrated in list 146 isexemplary only and various other information may alternatively be used.For example, center coordinate 148 may not be included. By way ofanother example, a region shape other than rectangular may be used, suchas a circle, ellipse, triangle, pentagon, hexagon, or free-form shapes.

Tracking list 146 records both faces and face candidates, which can bedistinguished from each other in a variety of manners. For example, twosub-lists (one identifying faces and the other identifying facecandidates) may be maintained, or an additional field may be added tolabel each field as either a face or a face candidate, or it may beinherent in the time since last verification 154 (e.g., if this is valueis blank it means that the region has not yet been verified as includinga face and thus is a face candidate). Alternatively, multiple lists maybe included rather than the single list 146 (e.g., one list for facesand another list for face candidates).

During operation, detection and tracking module 132 analyzes content ona frame by frame basis. For each frame, module 132 activates theauto-initialization module 140 which operates to detect candidates fornew face regions. Each such candidate is a region of the video contentthat potentially includes a new face (that is, a face that is notcurrently being tracked). Once detected, a candidate region is passed tohierarchical verification module 142, which in turn verifies whether thecandidate region does indeed include a face. Hierarchical verificationmodule 142 generates a confidence level for each candidate anddetermines to keep the candidate as a face region if the confidencelevel exceeds a threshold value, adding a description of the region totracking list 146. If the confidence level does not exceed the thresholdvalue, then hierarchical verification module 142 discards the candidate.

Multi-cue tracking module 144 tracks each of the regions identified intracking list 146. Tracking module 144 uses various visual cues to trackregions from frame to frame in the content. Each of the faces in aregion being tracked is an image of at least a portion of a person.Typically, people are able to move while the content is being generated,such as to stand up, sit down, walk around, move while seated in theirchair, and so forth. Rather than performing face detection in each frameof the content, module 132 tracks regions that include faces (oncedetected) from frame to frame, which is typically less computationallyexpensive than face detection.

In addition to being tracked, each region including a face from trackinglist 146 is repeatedly re-verified by hierarchical verification module142. Multi-cue tracking module 144, or alternatively hierarchicalverification module 142, may determine when a region is to bere-verified by module 142. Regions may be re-verified at regular orirregular intervals. When re-verifying a region, hierarchicalverification module 142 generates a new confidence level for the regionand compares the confidence level to the threshold value. If the newconfidence level exceeds the threshold value, then the time since lastverification 154 for the region is reset and the region is left intracking list 146. However, if the new confidence level does not exceedthe threshold value, then the region is deleted from tracking list 146.

It should be noted that situations can arise where multi-cue trackingmodule 144 loses its tracking. Hierarchical verification module 142resolves these situations by identifying when tracking of a regionincluding a face has been lost (e.g., a confidence level for the regionis low). This allows auto-initialization module 140 to re-detect theregion and tracking of the re-detected region to proceed.

Auto-Initialization

Auto-initialization module 140 uses one or more techniques to detectcandidates for new face regions. These techniques include motion-basedinitialization, audio-based sound source location, and fast facedetection. A motion-based initialization module 156 detects motion usingthe inter-frame difference (the difference between two or more frames ofthe video content) and determines whether the areas in which motion isdetected include a face. An audio-based initialization module 158analyzes the audio content corresponding to the video content, detects adirection from which sound is received, and searches the region of thevideo content in that direction to determine whether a region(s) in thedirection from which sound is received includes a face. Modules 156 and158 both operate to analyze each frame of video content. Alternatively,one of the modules 156 and 158 may operate on a particular frame ofvideo content only if the other module 156 or 158 fails to detect anyfaces.

Fast face detection module 160 operates when there is no motion or audioin the frame of the video content. Alternatively, module 160 may operatewhen there is motion and/or audio in the frame, but when neither module156 nor module 158 detects a face (or alternatively regardless ofwhether module 156 or 158 detects a face). Fast face detection module160 uses a fast face detector to analyze the frame of the video contentand detect faces in the frame. Lost confidence region detection module162 operates when auto-initialization module 140 is notified thatre-verification of a region has resulted in a loss in confidence thatthe region includes a face. Even though confidence that a regionincludes a face has been lost, it is still likely that a face is nearthis region. Lost confidence region detection module 162 communicateswith each of modules 156, 158, and 160 to have the modules 156, 158, and160 analyze the area of the video content around this region to attemptto detect a face in the area. The exact size of the area around theregion can vary by implementation (e.g., in one exemplary implementationthe area may extend above and below the region by one-half the height ofthe region, and extend to the left and right of the region by one-halfthe width of the region.

FIG. 4 is a flowchart illustrating an exemplary process 200 fordetecting candidates for new face regions. The process of FIG. 4 iscarried out by auto-initialization module 140 of FIG. 3, and may beperformed in software.

Initially, a frame of audio/video content is received (202). This frameof content can be received from any of a wide variety of sources. Forexample, the frame of content may be captured by one or more capturedevices of system 130 of FIG. 3, or the content may be capturedelsewhere and communicated to system 130 (e.g., via a removable storagedevice, via a network or telephone line connection, etc.). Oncereceived, an attempt to detect motion in the frame is made by comparingpixels of the frame to corresponding pixels of the previous frame in theaudio/video content (act 204). If motion is detected, then motion-basedinitialization is performed to identify candidates for new face regionsin the frame (act 206). After any candidates for new face regions usingmotion-based initialization are identified in act 206 an attempt is madeto detect audio in the frame (act 208). If audio is detected, thenaudio-based initialization is performed to identify candidates for newface regions in the frame (act 210). Any identified candidates for newface regions based on the motion-based initialization and/or theaudio-based initialization are passed to the hierarchical verificationmodule 142 for face verification (act 212).

Returning to act 204, if no motion is detected in the frame then anattempt is made to detect audio in the frame (act 214). If audio isdetected, then audio-based initialization is performed to identifycandidates for new face regions in the frame (act 210), and processingproceeds to act 212. However, if no audio is detected, then a fast facedetector is used to identify candidates for new face regions (act 216).Any identified candidates for new face regions based on a fast facedetection are passed to the hierarchical verification module 142 forface verification (act 212).

The area of the frame in which attempts are made to detect motion oraudio, or in which the fast face detector is used, can vary based on thesituation. In the situation where tracking list 146 includes no faces orface candidates, then the area of the frame is the entire frame. Insituations where tracking list 146 includes one or more faces or facecandidates, then the area of the frame includes all those areas that arenot currently being tracked (that is, are not listed in tracking list146). In situations where lost confidence region detection module 162requests that a particular area be analyzed, then the area of the frameis that area identified by module 162.

Returning to FIG. 3, motion-based initialization module 156 analyzes aframe of video content by comparing pixels in the frame to thecorresponding pixels in the previous frame(s) and/or subsequent frame(s)and detects whether there is motion between/among the frames at eachpixel. A moving individual is deemed to be in the foreground of thevideo content, and module 156 attempts to identify the shape of thismoving foreground. If the shape is similar to a human upper bodysilhouette (a smaller head on top of a larger shoulder), then the shapeis determined to be a face candidate.

FIG. 5 is a flowchart illustrating an exemplary process 240 foridentifying candidates for new face regions using motion-basedinitialization. The process of FIG. 5 is carried out by motion-basedinitialization module 156 of FIG. 3, and may be performed in software.

Initially, a determination is made as to whether there is motion at eachpixel (act 242). This determination is made for each pixel of a frame bycomparing the pixel to the corresponding pixel of the previous frame.The comparison can be made by, for example pixel intensity (e.g., graylevel) or color values. Various conventional filters may also be appliedto the pixels before being compared. The video content can be viewedusing a conventional 2-dimensional (x,y) coordinate system of pixels. Apixel in one frame at a particular coordinate location corresponds to apixel in another frame that is at that same coordinate location. Eachpixel in the area of the frame being analyzed has a frame differencegenerated as follows:

${D_{t}\left( {x,y} \right)} = \left\{ \begin{matrix}{1,} & {{{I_{t}\left( {x,y} \right)} - {I_{t - 1}\left( {x,y} \right)}} > d_{th}} \\{0,} & {otherwise}\end{matrix} \right.$where D_(t)(xy) is the frame difference between the pixel at location(x,y) in the image at frame t and the pixel at location (x,y) in theimage at frame t−1, I_(t)(x,y) is the pixel at location (x,y) in theimage at frame t, I_(t−1)(x,y) is the pixel at location (x,y) in theimage at frame t−1, and d_(th) is the threshold to decide if a pixel isa motion pixel. The exact value of d_(th) can vary by implementation,such as based on whether the frames are color or gray scale, what (ifany) filtering has been done, etc. As one particular example, a value of20 could be used for d_(th) if the pixels are 256-level gray scale.

Alternatively, the frame difference may be generated based on three ormore frames rather than just two frames. In one implementation, threeframes (e.g., I_(t−1), I_(t), I_(t+1)) are used to detect moving pixels.Only the pixels that have a large frame difference (e.g., greater thand_(th)) in both I_(t)(x,y)-I_(t−1)(x,y) and I_(t+1)(x,y)-I_(t)(x,y) arethe moving pixels.

Given the frame differences, the sum of the frame differences of eachpossible segment of each horizontal line of the image in the frame areabeing analyzed is generated (act 244). The image in the frame area beinganalyzed includes multiple horizontal lines. Each horizontal row ofpixels may be such a line, or alternatively every n^(th) (e.g., second,third, fourth, fifth, etc.) horizontal row of pixels may be such a line.Numerous segments of each such line exist, having different beginningand ending points on the line. The sum of the frame differences alongthe possible segments is used to attempt to identify the most likelyforeground segment in the area being analyzed. This is illustrated inadditional detail in FIG. 6.

FIG. 6 illustrates an exemplary image for a frame of video content. Animage 270 is illustrated including two regions 272 and 274 being alreadytracked as containing faces or face candidates, and the remaining area276 being analyzed for candidates for new face regions. Assuming thatthe image includes an individual 278, two horizontal lines 280 and 282will intersect image 278 at beginning points i and ending points j. Allof the pixels between points i and j on a particular line li should beon the foreground, and the boundaries between two consecutive horizontallines should also have a smoothness constraint—they tend to have asimilar center and similar width. The frame difference sums are used toidentify the portions of the horizontal lines li with beginning points iand ending points j.

For each horizontal line, the sum S of the frame difference of eachpossible segment on the horizontal line is generated as follows:

${{S\left( {i,j} \right)} = {{\sum\limits_{x = i}^{j}{{D\left( {x,y} \right)}\mspace{14mu} 0}} < i < j < N}},{y \in \left\lbrack {0,M} \right\rbrack}$where i is the starting point of the segment, j is the ending point ofthe segment, D(x,y) is the frame difference at location x,y along thesegment, N is the length of the horizontal line, and M is the number ofhorizontal lines.

In order to increase the speed at which the sum for all possible i and jcan be calculated, the following process is used. First, for every valuei that is between zero and N, inclusive, the following is generated:S(i,i)=D(i,y), i∈[0,N]Then, compute from k=1 to k=N,S(i,i+k)=S(i,i+k−1)+S(i+k,i+k), i∈[0,N−k]

Returning to FIG. 5, once the sum of the frame difference for eachpossible segment on the horizontal lines are generated, for eachhorizontal line the segment with the largest sum is selected as the mostlikely foreground segment on that line (act 246). Whether the segmentwith the largest sum is actually part of a candidate for a new faceregion also depends on smoothness constraints, as discussed below. Thesmoothest region of most likely segments is then determined (act 248).The smoothest region is generated by considering the smoothnessconstraint across all the horizontal lines. This is achieved as follows.The process begins with y=0 (the top horizontal line) withE^(o)(i⁽⁰⁾,j⁽⁰⁾)=S(i⁽⁰⁾,j⁽⁰⁾) and propagates to y=M (the bottomhorizontal line) by the following recursive function:

${E^{o}\left( {i,^{(y)},j^{(y)}} \right)} = {{S\left( {i^{(y)},j^{(y)}} \right)} + {\max\limits_{i^{({y - 1})},{j^{({y - 1})} \in {\lbrack{0,N}\rbrack}}}\left( {{E^{o}\left( {i^{({y - 1})},j^{({y - 1})}} \right)} + {C\left( {\begin{bmatrix}i^{({y - 1})} \\j^{({y - 1})}\end{bmatrix},\begin{bmatrix}i^{(y)} \\j^{(y)}\end{bmatrix}} \right)}} \right)}}$where i^((y)) and j^((y)) are the boundaries on (y)th horizontal line,and N is the width of the image. The C(.,.) parameter is the smoothnessenergy term. The C(.,.) parameter gives a large penalty to non-smoothboundaries between successive lines, and is defined as follows:

${C\left( {\begin{bmatrix}i^{({y - 1})} \\j^{({y - 1})}\end{bmatrix},\begin{bmatrix}i^{(y)} \\j^{(y)}\end{bmatrix}} \right)} = {{c_{c} \cdot {{\frac{i^{(y)} + j^{(y)}}{2} - \frac{i^{({y - 1})} + j^{({y - 1})}}{2}}}} + {c_{w} \cdot {{\left( {j^{(y)} - i^{(y)}} \right) - \left( {j^{({y - 1})} - i^{({y - 1})}} \right)}}}}$where c_(c) is the penalty coefficient for non-smoothness of the segmentcenter while a c_(w) is the penalty coefficient for non-smoothness ofthe segment width. Different values for the penalty coefficients c_(c)and c_(w) can be used, and in one exemplary implementation each of thec_(c) and c_(w) values is 0.5.

The smoothest region can then be obtained by determining:

$\max\limits_{i^{(M)},{j^{(M)} \in {\lbrack{0,N}\rbrack}}}\left( {E^{o}\left( {i^{(M)},j^{(M)}} \right)} \right)$Given this smoothest region, a back trace to find the boundaries on allhorizontal lines can be performed.

Given the smoothest region, a check is made as to whether the regionresembles a human upper body (act 250). In the illustrated example, thehuman upper body includes a smaller head on top of a larger shoulder.So, a check is made as to whether the smoothest region has anapproximately elliptical portion (the head) located above a widerportion (the shoulder). In one implementation, this check is made byfirst detecting the position of the neck by finding the largest changeof the width on neighboring horizontal lines. Then, a check is made asto whether the region above the neck (the head region) has a smalleraverage width than the lower region (the shoulder region). A check isalso made as to whether the width to height ratio of the head region isapproximately 1:1.2. If all of these checks are true, then the detectedregion is determined to resemble a human upper body silhouette.

If the region does resemble a human upper body, then the portion of theregion including the head (but excluding the shoulders) is extracted(act 252) and identified as a candidate for a new face region (act 254).This extracted region may be the approximately elliptical region of thehuman head or an area around the head (e.g., a rectangular region aboutthe head). However, if the region does not resemble a human upper body,then no candidates for a new face region are detected from the frame(act 256).

In one implementation, the process of FIG. 5 is repeated if a candidatefor a new face region is identified in act 254 and if there are anyadditional regions in the frame (not counting the candidate identifiedin act 254 or any other faces or face candidates). This allowsadditional candidates for new face regions to be identified in theframe.

Returning to FIGS. 3 and 4, audio-based initialization module 158analyzes a frame of audio/video content (act 210 of FIG. 4) by using asound source locator to detect a direction from which sound is received.Module 158 assumes that this sound may be human speech, and thus isindicative of a region of the video content that may include a candidatefor a face region. The direction from which sound is received can bedetermined in a wide variety of different manners. In oneimplementation, one or more microphone arrays capture sound and one ormore sound source localization algorithms are used to determine whichdirection the sound came from. A variety of different conventional soundsource localization algorithms can be used, such as well-knowntime-delay-of-arrival (TDOA) techniques (e.g., the generalizedcross-correlation (GCC) approach).

In situations where there is no video content, face detection can beaccomplished by proper placement of multiple microphones. Using three ormore microphones, at least two of which are located on differenthorizontal planes and at least two of which are located on differentvertical planes, an (x,y) coordinate can be determined for the soundsource. For example, two microphones may be located in the verticalplane and two microphones may be located in the horizontal plane. Any ofa variety of conventional sound source localization algorithms can thenbe used to determine an (x,y) location of the sound source, which ispresumed to be an individual's mouth. This sound source location itselfcan be treated as the detected face region (given that the speaker'smouth is part of the speaker's face), or alternatively the location maybe expanded (e.g., increased by two or three percent) and the expandedlocation used as the detected face region.

Given an area of the image that corresponds to the direction from whichsound is received, initialization module 158 analyzes that area andattempts to fit a skin color model to the image in that area. If thisattempt is successful, then the area to which the skin color model isfit is identified as a candidate for a new face region. In oneimplementation, the skin color model is a HSV (Hue-Saturation-Value)color space model, with numerous skin color training data being used totrain the model). It should be noted that, because the audio alreadyindicates that there is a face in the region, a coarse detection process(e.g., a skin color model) can be used to locate the face.

In situations where no video content is available, module 158 relies onthe sound source location determination without use of the skin colormodel (as there is no video content to which the skin color model can beapplied).

Fast face detection module 160 uses a fast face detector to detect aface(s) with the areas of the image of the frame. The fast face detectorused by detection module 160 can be different than the face detectorused by hierarchical verification module 142 as discussed in more detailbelow. For computation and accuracy tradeoffs, the face detector used bymodule 160 is faster, but less accurate, than the face detector used byhierarchical verification module 142; however, modules 160 and 142 maybe based on the same face detection algorithm, but use differentparameters or thresholds in order to increase the speed of detection bymodule 160 relative to the speed of detection by module 142.Alternatively, modules 160 and 142 may be based on two different facedetection algorithms. The detector used by detection module 160 istypically faster than the detector used by hierarchical verificationmodule 142.

A wide variety of face detection algorithms can be used as the basis forfast face detection module 160, with a primary characteristic of thealgorithm(s) used being its (their) speed. The goal of fast facedetection module 160 is to detect faces quickly, at the expense ofaccuracy if necessary. The face detection may be frontal-face only, oralternatively may be multi-view (and not limited to frontal-facedetection). An example of such an algorithm is described in P. Viola andM. J. Jones, “Robust real-time object detection”, Technical ReportSeries, Compaq Cambridge Research laboratory, CXRL 2001/01, February2001. Another example of such an algorithm is similar to that discussedin P. Viola and M. J. Jones, except that stages of detectors are usedstarting with a detector that covers a wide range of degrees of view,and advancing to a set of multiple detectors each covering a narrowerrange of degrees of view. Objects are passed from one stage of detectorsto another, with each detector classifying the object as either a faceor a non-face. As soon as an object is classified as a non-face by anydetector it is dropped from the process—only those objects that passthrough and are classified by all stages of detectors as faces areidentified as faces.

Thus, using one or more of the motion-based initialization, audio-basedsound source location, and fast detection techniques,auto-initialization module 140 detects candidates for new face regions.These candidates are then passed to hierarchical verification module 142for verification as to whether the candidates actually include a face.It should be noted that not all frames will include new faces, and thusauto-initialization module 140 may not detect any candidates for newface regions in a frame even if using all of the above-referencedtechniques.

Hierarchical Verification

Hierarchical verification module 142 of FIG. 3 verifies candidate faceregions identified by auto-initialization module 140. Additionally,detection and us tracking module 132 accounts for the possibility thatmulti-cue tracking module 144 may lose track of objects duringoperation. This may occur for a variety of reasons, such as occlusions(e.g., when another participant walks between the video capture deviceand the individual being tracked) or sudden lighting changes.Hierarchical verification module 142 re-verifies, at regular orirregular intervals, each object being tracked and downgrades objectsfrom faces to face candidates as appropriate. The length of theintervals can vary, based on how accurate the tracking is desired to be(shorter intervals tend to improve the accuracy), the amount ofcomputing power available (depending on the type of verifying, thetracking may take less computing power than re-verifying), and thecomputational expense of the verification module(s).

In one implementation, hierarchical verification module 142 verifiesobjects as faces and identifies an object as either a face or not aface. Alternatively, verification module 142 may also outputprobabilistic verification results based on different features (e.g.,audio, color histogram distance, edge detection results around theboundary, face detection results, etc.). In so doing, the outputprobabilistic verification results can be combined with the weightingscheme of particle-filtering discussed in more detail below.

Because of computation considerations, hierarchical verification module142 uses a multilevel hierarchical process to verify an object includesa face. The verification process is a coarse to fine process startingwith faster, but less It accurate, verification and rising to slower,but more accurate, verification if needed. In the illustrated example,the hierarchical process includes two levels. Alternatively, three ormore levels may be included in the hierarchical process.

Hierarchical verification module 142 of FIG. 3 includes a fastcolor-based verification module 164, and a multi-view face detectionmodule 166. Verification module 142 assumes that an object typicallydoes not change color significantly during successive frames.Color-based verification module 164 verifies objects based on thesimilarity between the color histogram of the object in the currentframe and the estimated color histogram of the object in the previousframes. When the similarity is high, it is assumed that no loss oftracking has occurred and multi-view face detection module 166 need notbe invoked. However, when the similarity is low a loss of tracking mayhave occurred, so the object is downgraded from a face to a facecandidate and passed to multi-view face detection module 166. If themulti-view face detection module 166 verifies the object as a face, theobject is upgraded from face candidate to face. However, if detectionmodule 166 does not verify the object as a face, the object is deletedfrom tracking list 146.

In one implementation, color-based verification module 164 performs itsverification for each frame, while multi-view face detection module 166performs its verification less frequently. As an example, multi-viewface detection module 166 may perform its verification once every fewseconds, although different intervals may also be used based on thevarious factors discussed above.

FIG. 7 is a flowchart illustrating an exemplary process 320 forperforming hierarchical verification. Process 320 is performed byhierarchical verification module 142 of FIG. 3, and may be performed insoftware.

Initially, an image of the interested area is obtained (act 322). Theinterested area may be a candidate region, identified byauto-initialization module 140, or a region for re-verification.Hierarchical verification module 142 may be passed the entire frame withan indication of the area to be analyzed, or alternatively only theportion of the frame that includes the area to be analyzed. Oncereceived, a fast color-based verification is used to verify whether aface is in the area (act 324).

The fast color-based verification of act 324 is illustrated inadditional detail with reference to FIG. 8. The process 324 of FIG. 8 isperformed by fast color-based verification module 164 of FIG. 3, and maybe performed in software. Initially, a color histogram (q_(t)(x)) of theobject in the current frame t is generated (act 362). An estimated colorhistogram (p_(t−1)(x)) of the object in previous frames is alsogenerated (act 364). The estimated color histogram p_(t−1)(x) isgenerated as follows:p _(t−1)(x)=α·q _(t−1)(x)+(1−α)·p _(t−2)(x)where α represents a weight, q_(t−1)(x) is the color histogram of theobject in the previous frame t−1, and p_(t−2)(x) is the estimated colorhistogram generated for the object in the previous frame t−1. A widerange of values for a can be used in different implementations, theexact value being selected as a tradeoff between trust of the historyand trust of the current frame (e.g., in one exemplary implementation,the value of a can range from 0.25 to 0.75). The estimated colorhistogram p_(t−1)(x) for the object is thus updated based on the colorhistogram of the object in each frame.

The similarity of the two histograms is then determined (act 366). Todetermine the similarity measure of the two histograms q_(t)(x) andp_(t−1)(x), the well-known Bhattacharyya Coefficient is used as follows:ρ(p _(t−1)(x),q _(t)(x))=∫√{square root over (p _(t−1)(x)·q_(t)(x))}{square root over (p _(t−1)(x)·q _(t)(x))}dxwhere ρ represents the probability of classification error instatistical hypotheses testing—the larger the probability of error, themore similar the two distributions is are. The value of ρ ranges fromzero to one, with one meaning the two histograms are the same and zeromeaning the two histograms are totally different. This similaritymeasure is also referred to herein as a confidence level. Alternatively,other well-known similarity measures may be used, such as K-Ldivergence, histogram intersection, and so forth.

A check is then made as to whether the similarity between the twohistograms exceeds a threshold amount (act 368). If the difference isgreater than the threshold amount then the face is verified (act 370);that is, the object is verified as including a face. However, if thedifference is not greater than the threshold amount then the face is notverified (act 372); that is, the object is not verified as including aface. Different thresholds can be used in different implementations. Inone exemplary implementation, the threshold value can range from 0.90 to0.95, and in one particular implementation is 0.94.

Returning to FIG. 7, processing proceeds based on whether the face isverified (act 326). If the face is verified, then it is upgraded from aface candidate to a face (if not already a face) (act 328), and thehierarchical verification process is completed (act 330) and no moreverification is performed for the interested area at this time. However,if the face is not verified, then the face is downgraded from a face toa face candidate (if currently a face) (act 332). The object includingthe face is then passed to multi-view face detection module 166 of FIG.3, which uses multi-view face detection to verify whether a face is inthe area (act 334).

Multi-view face detection module 166 uses one or more detectionprocesses that attempt to detect human faces in different poses or frommultiple views (that is, to detect the faces even though the head may betilted, rotated away from the image capturing device, etc.). Any of awide variety of face detection techniques can be used by multi-view facedetection module 166.

One such multi-view face detection process is a kernel machine basedprocess, discussed in additional detail in S. Z. Li, Q. D. Fu, L. Gu, B.Scholkopf, Y. M. Cheng, H. J. Zhang., “Kernel Machine Based learning forMulti-View Face Detection and Pose Estimation,” Proceedings of 8th IEEEInternational Conference on Computer Vision, Vancouver, Canada, Jul.9-12, 2001. A summary of this detection process follows.

Let Ip∈

^(N) be a windowed grey-level image or appearance of a face. Assume thatall left rotated faces (those with view angles between 91° and 180°) aremirrored to right rotates so that every view angle is between 0° and90°. Quantize the pose into a set of L discrete values (e.g., chooseL=10 for 10 equally spaced angles 0° to 90°, with 0° corresponding tothe right side view and 90° to the frontal view).

Assume that a set of training face images are provided for the learning.The images Ip are subject to changes not only in the view, but also inillumination. The training set is view-labeled in that each face imageis manually labeled with its view value as close to the truth aspossible, and then assigned into one of L groups according to thenearest view value. This produces L view-labeled face image subsets forlearning view-subspaces of faces. Another training set of nonface imagesis also used for training face detection.

Now, there are L+1 classes indexed in the following by l, with l∈{0,1, .. . , L−1} corresponding to the L views of faces and l=L correspondingto the nonface class. Two tasks, face detection and pose estimation, areperformed jointly by classifying the input Ip into one of the L+1classes. If the input is classified into one of the L face classes , aface is detected and the corresponding view is the estimated pose;otherwise, the input pattern is considered as a nonface pattern.

The learning for face detection and pose estimation using kernelmachines is carried out in two stages: one for kernel principalcomponent analysis (KPCA) view-subspace learning, and one for kernelsupport vector classifier (KSVC) classifier training. Stage 1 trainingaims to learn the L KPCA view-subspaces from the L face view subsets.One set of kernel principal components (KPCs) are learned form each viewsubset. The most significant components (e.g., the top 50) are used asthe basic vectors to construct the view-subspace. The learning in thisstage yields L view-subspaces, each determined by a set of supportvectors and the corresponding coefficients. The KPCA in each viewchannel effectively performs a nonlinear mapping from the input imagespace to the output KPCA feature space (having the same dimension as thenumber of components in the most significant components).

Stage 2 aims to train L KSVC's to differentiate between face and nonfacepatterns for face detection. This uses a training set consisting of anonface subset as well as L view face subsets. Once KSVC is trained foreach view to perform the L+1-class classification based on the featuresin the corresponding KPCA subspace. The projection onto the KPCAsubspace of the corresponding view is used as the feature vector. Thewell-known one-against-the-rest method is used for solving themulti-class problem in a KSVC. Stage 2 gives L KSVCs.

In the testing stage, a test sample is presented to the KPCA featureextractor for each view l to obtain the feature vector for that view.The corresponding KSVC of that view calculates an output vectory_(t)=(y_(l) ^(c)|c=0, . . . ,L) as the responses of the L+1 classes tothe input. This is done for all the L view channels so that L suchoutput vectors {y_(t)|l=0, . . . , L−1) are produced. The value y_(l)^(c) is the evidence for the judgment that the input Ip belongs to classc in terms of the features in the l-th view KPCA subspace. The finalclassification decision is made by fusing the evidences from all the Lview channels. One way for the fusing is to sum the evidences; that is,for each class c=0, . . . ,L, the following is calculated:

${y^{c}({Ip})} = {\sum\limits_{t = 0}^{L - 1}y_{l}^{c}}$This calculation gives the overall evidence for classifying Ip intoclass c. The final decision is made by maximizing the evidence: Ipbelongs to c* if c*=argmax_(c) y^(c)(Ip).

Continuing with FIG. 7, processing then proceeds based on whether theface is verified by the multi-view face detection (act 336). If the faceis verified, then the face is upgraded from a face candidate to a face(act 328) and the hierarchical verification process is completed (act330). However, if the face is not verified, then the candidate isdropped from tracking list 146 of FIG. 3 (act 338), and the hierarchicalverification process is completed (act 330).

In situations where there is no video content to be analyzed forhierarchical verification, audio cues alone can be used for verificationwhen appropriate. For example, audio cues alone may be used when theperson whose face is being tracked is talking continuously, or whenwell-known audio speaker based identification is performed (therebyallowing sound sources to be tied to individual speaker's voices, andverification performed by determining whether the voice coming from aparticular sound source location matches the same speaker identificationas was previously received from that sound source location).

Multi-Cue Tracking

Once a face is detected in a frame of video content, the face is trackedby multi-cue tracking module 144 of FIG. 3 in subsequent frames of thevideo content. The participant whose face is being tracked may moveabout, and thus the location of the face may be different in differentframes of the video content. Furthermore, the participant may rotate hisor her head (e.g., so that his or her face no longer looks directly atthe video capture device), various occlusions may occur (e.g., theparticipant may pass his or her hand in front of his or her face),lighting may change, and so forth. Multi-cue tracking module 144attempts to account for these various changes that may occur from frameto frame. Additionally, because of these changes, some cues may becomeunreliable to track. Multi-cue tracking module 144 also attempts toaccount for these changes in cue reliability that may occur from frameto frame.

Various cues are used by tracking module 144 in tracking a face. In oneimplementation, these tracking cues include the shape of the face (whichis modeled as an ellipse), motion, edges, foreground color, andbackground color. Alternatively, one or more of these cues may not beused, or additional cues may be used, such as audio cues.

Multi-cue tracking module 144 may use audio cues to assist in tracking(or as the sole basis for tracking) when audio content is available. Theaudio-based tracking is performed based on sound source locationprocess(es), and is performed in the same manner as audio-baseddetection is performed by audio-based initialization module 158 of FIG.3 discussed above.

FIG. 9 is a flowchart illustrating an exemplary process 400 forperforming multi-cue tracking. Process 400 is performed by multi-cuetracking module 144 of FIG. 3, and may be performed in software.

Initially, a prediction is made as to where the object will be in thecurrent frame t based on the tracking results from the previous framet−1 and the object's dynamics (modeled by the well-known Langevinprocess, and discussed in more detail below) (act 402). Observations arecollected along a set of normal lines of the predicted contour of theobject (act 404), and an observation likelihood function is evaluatedfor every pixel on the normal lines (act 406). The state transitionprobabilities from frame t−1 to frame t are evaluated (act 408), and thebest contour with respect to the given observations is determined (act410). The best ellipse is fitted to the image of frame t based on thedetected contour (act 412), and the model is adapted for use with thenext frame t+1 (act 414).

Multi-cue tracking module 144 includes various modules for performingthe acts of FIG. 9. In the illustrated example, tracking module 144includes: an observation likelihood module 168, a smoothness constraintmodule 170, a contour selection module 172, and a model adaptationmodule 174.

Multi-cue tracking module 144 focuses on tracking human heads, whichhave an elliptical shape (approximately 1:1.2). The human head for theface being tracked is represented by a model that is an ellipse havingvarious tracking cues. When analyzing an image of a frame of videocontent, the model is compared to various locations of the image and adetermination made as to which location most closely matches the model.This location that most closely matches the model is selected as theface in the new frame.

FIG. 10 illustrates this modeling and comparison in additional detail.In FIG. 10, a solid curve 422 represents a predicted contour of a humanhead in a particular frame t based on the tracking results from theimmediately preceding frame t−1. The dashed curve 424 represents thetrue contour of the human head in frame t. A set of measurements arecollected along multiple (M) normal lines 426 of the predicted contour422. The point 428 (c(φ)) is the true contour point on the φ^(th) normalline. The point 430 (ρ_(φ)(N)) is the predicted contour point on theφ_(th) normal line. Multi-cue tracking module 144 attempts to locate thetrue contour 424 by having as many contour points as possible on thepredicted contour 422 be the same as the contour points on the truecontour line 424.

Observation likelihood module 168 of FIG. 3 generates a value ρ_(φ)(λ),which denotes the image intensity at pixel λ on line φ, as follows:ρ_(φ)(λ)=I(x _(λφ) ,y _(λφ))where φ ranges from 1 to M (the total number of normal lines 246) and λranges from −N to N along the normal line (each normal line has 2N+1pixels), x_(λφ), y_(λφ) is the corresponding image coordinate of thepixel λ on the φ^(th) normal line, and I(x_(λφ),y_(λφ)) is the imageintensity at point (x_(λφ),y_(λφ)).

To detect the contour points, different cues (e.g., edge intensity,color model of the foreground and background) and prior constraints(e.g. contour smoothness constraint) can be integrated by using a HiddenMarkov Model (HMM). Hidden Markov Models are well-known to those skilledin the art, and thus will not be discussed further except as theypertain to the automated tracking of multiple individuals as describedherein. The hidden states of the HMM are the true contour points on eachnormal line, (denoted as s={s₁. . . ,s_(φ), . . . ,s_(M)}). Theobservations of the HMM, O={O₁, . . . ,O_(φ), . . . O_(M)}, arecollected along each normal line φ. A HMM is specified by the number ofstates (in our case, 2N+1), the observation model P(O_(φ)|s_(φ)), andthe transition probability p(s_(φ)|s_(φ−1)).

Observation likelihood module 168 proceeds to generate a multi-cueobservation likelihood function as follows. The observation on line φ(represented as O_(φ)) can include multiple cues, e.g., pixel intensity(i.e., ρ_(φ)(λ), λ∈[−N, N]) and edge intensity (i.e., z_(φ)) along theline. The observation likelihood model of the edge detection resultsz_(φ) can be derived using any of a variety of conventional edgedetection processes, such as the well-known Sobel edge detector or Cannyedge detector. Due to noise and image clutter, there can be multipleedges along each normal line φ. The value J is used to represent thenumber of detected edges (z_(φ)=(z₁,z₂, . . . ,z_(J))). Of the Jdetected edges, at most one is on the true contour line 424 of FIG. 10.We can therefore define J+1 hypotheses:H _(o) ={e _(j) =F:j=1, . . . ,J}H ₁ ={e _(j) =T,e _(k) =F:k=1, . . . ,J,k≠j}where e_(j)=T means that the jth edge is associated with the truecontour line, and e_(j)=F means that the jth edge is not associated withthe true contour line. Hypothesis H₀ therefore means that none of theedges is associated with the true contour line.

Assuming that the image clutter is a well-known Poisson process alongthe line with spatial density γ and the true target measurement isnormally distributed with standard deviation σ_(z), the edge likelihoodmodel is obtained as follows:

${p\left( {{z_{\phi}❘s_{\phi}} = \lambda_{\phi}} \right)} \propto {1 + {\frac{1}{\sqrt{2\pi}\sigma_{z}q\;\gamma}{\sum\limits_{m = 1}^{J}{\exp\left( {- \frac{\left( {z_{m} - \lambda_{\phi}} \right)^{2}}{2\sigma_{z}^{2}}} \right)}}}}$where q is the prior probability of hypothesis H₀.

In addition to the edge likelihood model, other cues about the regionproperties of the foreground and background, e.g., mixture color models,are integrated into the HMM framework. Let p(v|FG) and p(v|BG) representthe color distribution for the foreground (FG) and background (BG),respectively. The posterior probabilities P(BG|ν) and P(FG|ν) can bederived as follows:

$\begin{matrix}{{{P\left( {{BG}❘v} \right)} = \frac{p\left( {v❘{BG}} \right)}{{p\left( {v❘{BG}} \right)} + {p\left( {v❘{FG}} \right)}}}{{P\left( {{FG}❘v} \right)} = \frac{p\left( {v❘{FG}} \right)}{{p\left( {v❘{BG}} \right)} + {p\left( {v❘{FG}} \right)}}}} & (1)\end{matrix}$

If s_(φ)=λ_(φ) is the contour point on line φ, then the segment [−N,s_(φ)] is on the foreground and the segment [s_(φ)+1, N] is on thebackground. Combining the edge likelihood model and the color posteriorprobabilities results in the following multi-cue observation likelihoodfunction for the HMM:

$\begin{matrix}{{P\left( {O_{\phi}❘s_{\phi}} \right)} = {{p\left( {z❘s_{\phi}} \right)} \cdot {\prod\limits_{i = {- N}}^{s_{\phi}}\;{{P\left( {{{BG}❘v} = {\rho_{\phi}(i)}} \right)} \cdot {\prod\limits_{i = {s_{\phi} + 1}}^{N}\;{P\left( {{{FG}❘v} = {\rho_{\phi}(i)}} \right)}}}}}} & (2)\end{matrix}$Other cues, such as audio cues (e.g., based on sound source location andlikelihood of a sound coming from a particular location) can also beintegrated in analogous manners. In situations where there is no videocontent for analysis, solely the audio cues are used. Alternatively, inaddition to or in place of such audio queues, audio can be used asproposal functions with unscented particle-filtering, discussed in moredetail below.

Another component in HMM is the transition probability, which determineshow a state at time t−1 transits to another state at time t. Smoothnessconstraint module 170 of FIG. 3 derives the transition probability.

In order to obtain a smooth contour, transition probabilities are usedto encode a smoothness constraint and penalize roughness. Referring toFIG. 10, it can be seen that when the normal lines 426 are dense (e.g.,on the order of 30 normal lines), the points of true contour line 424 onadjacent normal lines 426 tend to have the same displacement from thepredicted contour line 422 (indexed as zero on each normal line). Thiscorrelation is used to help obtain a smooth contour.

In HMM, given current state s_(φ), the current observation O_(φ) isindependent of previous state s_(φ−1) and previous observation O_(φ−1).In addition, because of the Markovian property, we have p(s_(φ)|s₁,s₂, .. . s_(φ−1))=p(s_(φ)|s_(φ−1)).

The contour smoothness constraint can then be captured by the statetransition p(s_(φ)|s_(φ−1)) as follows:p(s _(φ) |s _(φ−1))=c·exp(−(s _(φ) −s _(φ−1)) ²σ_(s) ²)   (3)where c is a normalization constant and σ_(s) is a predefined constantthat regulates the smoothness of the contour. This transitionprobability penalizes sudden changes of the contour points betweenadjacent lines, hence resulting in a smooth contour. The best contourcan then be obtained by contour selection module 172.

The transition probability generated by smoothness constraint module 170based on calculation (3) above considers the contour point withoutregard for other pixels on the normal lines. Alternatively, smoothnessconstraint module 170 uses a JPDAF (joint probability data associationfilter)—based method to encode not only the contour smoothnessconstraint, but also the region smoothness constraint observed onmultiple (e.g., all) the pixels on the normal lines. In the illustratedexample, a JPDAF process based on dynamic programming is used to improvereal-time performance.

Under typical conditions, pixel intensity values of parts of the humanbody (e.g., face or head) change smoothly inside their regions. It istherefore a reasonable assumption that in human tracking, the foregroundand background have smooth region properties so that the measurements ontwo adjacent lines are similar. Let s_(φ) and s_(φ+1) be the contourpoints on line φ and line φ+1, respectively. These two contour pointssegment the two lines into foreground segments and background segments.Based on the region smoothness assumption, not only should s_(φ) ands_(φ+1) be close to each other, but all the other pixels on the twolines should also match well. To obtain the region smoothnessconstraint, a joint probability data association filter is used toconduct the line matching. That is, it is not a single point to singlepoint matching problem, but rather a (2N+1) points to (2N+1) pointsmatching problem. By considering all the pixels along the linestogether, more robust matching results can be obtained. The transitionprobabilities based on this JPDAF process are therefore typically moreaccurate. Let D^(F)(i,j) and D^(B)(i,j) be the matching distances of theforeground ([−N, i] on line φ and [−N,j] on line φ+1) and background([i+1, N] on line φ and [j+1, N] on line φ+1), respectively. Atransition probability can then be defined as follows to replace the onediscussed above with reference to calculation (3):log(p(s ₂ |s ₁)=D ^(F)(s ₁ ,s ₂)+D ^(B)(s ₁ ,s ₂)+(s ₂ −s ₁)²/σ_(s) ²  (4)

The region smoothness concept can be illustrated by a synthesized imageillustrated in FIG. 11. There are two regions illustrated: a rectangularregion 460 that represents background clutter and a roughly circularregion 462 that represents the object. Two adjacent normal lines 464 and466 are also illustrated. Points a and b are detected edge points online 464, while points c and d are detected edge points on line 466. Thegoal is to find where the contour points are on these two lines 464 and466. The measurements of intensities along the two lines 464 and 466 areillustrated in FIG. 12. Measurement 482 represents the intensities alongline 464, while measurement 484 represents the intensities along line466. Measurements 482 and 484 are similar to each other except for somedistortions. Based on the contour smoothness constraint only, thecontour from a to c and the contour from b to c have almost the sameamount of smoothness energy because |a−c|≅|b−c|. However, if we considerthe region smoothness assumption as well, the possible contour can be ador bc, but not ac or bd. The contour candidates ad and bc can further bediscriminated by HMM based on all the observation lines.

To get the new transition probabilities, the matching between all thepossible pairs of states ((2N+1)²) is calculated. FIG. 13 illustratescalculation of the matching distance graphically. Given lines 464 and466, the calculation of the matching distance can be explained in thefollowing recursive equation and can be seen in FIG. 13:

${D^{F}\left( {i,j} \right)} = {\min\left\{ \begin{matrix}{{D^{F}\left( {{i - 1},j} \right)} + {d\left( {{\rho_{1}(i)},{\rho_{2}(j)}} \right)}} \\{{D^{F}\left( {i,{j - 1}} \right)} + {d\left( {{\rho_{1}(i)},{\rho_{2}(j)}} \right)}} \\{{D^{F}\left( {{i - 1},{j - 1}} \right)} + {d\left( {{\rho_{1}(i)},{\rho_{2}(j)}} \right)}}\end{matrix} \right.}$where d(.,.) is the cost of matching two pixels. D^(F)(i,j) is the bestmatching distance between segment [−N, i] on line 464 and segment [−N,j] on line 466. Starting from D^(F)(0, j)=D^(F)(i,0)=0, where i,j∈[−N,N], use the above recursion to obtain the matching distance D^(F)(i,j)from i=−N to N and j=−N to N. An analogous process is gone through tocalculate the D^(B)(i,j), but starting from D^(B)(N,N)=0 to D^(B)(−N,−N). After obtaining all the matching distances, the state transitionprobabilities can be computed and contour tracking can be accomplishedby contour selection module 172 of FIG. 3, discussed in more detailbelow.

Given the observation sequence O={O_(φ)φ∈[1, M]} and the transitionprobabilities a_(i,j)=p(s_(φ+1)=j|s_(φ)=i), contour selection module 172determines the best contour found by finding the most likely statesequence s* using the well-known Viterbi algorithms as follows:

$s^{*} = {{\arg\;{\max\limits_{s}{P\left( {s❘O} \right)}}} = {\arg\;{\max\limits_{s}{P\left( {s,O} \right)}}}}$A value V(φ,λ) is defined as follows:

V(ϕ, λ) = max_(s_(ϕ − 1))P(O_(ϕ − 1), s_(ϕ − 1), s_(ϕ) = λ)Using the Markov conditional independence assumptions, V(φ,λ) can berecursively computed as follows:

${V\left( {\phi,\lambda} \right)} = {{P\left( {{O_{\phi}❘s_{\phi}} = \lambda} \right)} \cdot {\max\limits_{j}{{P\left( {s_{\phi} = {{\lambda ❘s_{\phi - 1}} = j}} \right)}{V\left( {j,{\phi - 1}} \right)}}}}$${j^{*}\left( {\phi,\lambda} \right)} = {{{P\left( {{O_{\phi}❘s_{\phi}} = \lambda} \right)} \cdot \arg}\;{\max\limits_{j}{{P\left( {s_{\phi} = {{\lambda ❘s_{\phi - 1}} = j}} \right)}{V\left( {j,{\phi - 1}} \right)}}}}$with the initialization V(1, λ)=max_(s1)P(O₁|s₁)P(s₁), where the initialstate probabilities P(s₁)=1/(2N+1), s₁∈[−N, N]. The term j*(φ,λ) recordsthe “best previous state” from state λ at line φ. Therefore, at the endof the sequence, max_(s)P(O,s)=max_(λ)V(M,λ) is obtained. The optimalstate sequence s* can be obtained by back tracking j*, starting froms_(M)*=argmax_(λ)V(M, λ), with s_(φ−1)=j*(s_(φ)*, φ).

Given the best state sequence s*={s₁*, . . . , s_(M)*}, thecorresponding image coordinate of the best contour point s_(φ)* on lineφ is denoted by [x_(φ), y_(φ)]. Because an ellipse is used as theparametric contour model, for each contour point [x_(φ), y_(φ)], thefollowing holds:ax _(φ) ² +by _(φ) ² +cx _(φ) y _(φ) +dx _(φ) +ey _(φ)−1=0

A matrix representation of these equations is:A·f=bwhere

$A = \begin{bmatrix}x_{1}^{2} & y_{1}^{2} & {x_{1}y_{1}} & x_{1} & y_{1} \\\vdots & \; & \; & \; & \vdots \\x_{M}^{2} & y_{M}^{2} & {x_{M}y_{M}} & x_{M} & y_{M}\end{bmatrix}$and b=[1, 1, . . . , 1]^(T). The parameters of the best−fit ellipsef*=[a, b, c, d, e]^(T) can be obtained by the least mean square (LMS)solution:f*=(A ^(T) A)⁻¹ A ^(T) b   (5)

The above ellipse representation f=[a, b, c, d, e]^(T) is convenientmathematically. But there is no clear physical interpretation of thefive parameters. In tracking, a different 5-element ellipserepresentation is normally used:θ=[x,y,α,β,Φ]where (x, y) is the center of the ellipse, α and β are the lengths ofthe major and minor axes of the ellipse, and φ is the orientation of theellipse. Because f and θ are two representations of the same ellipse,they are used interchangeably herein.

In a dynamic environment, both the object(s) being tracked and thebackground may gradually change appearance. Thus, model adaptationmodule 174 adapts the observation likelihood models dynamically. One wayto adapt the observation likelihood models is to completely trust thecontour returned by the Viterbi algorithm at frame t−1, and average allthe pixels inside and outside the contour to obtain the newforeground/background color model at frame t. However, if an erroroccurs at frame t−1, this procedure may adapt the model in the wrongway. Thus, model adaptation module 174 trains the observation models ina probabilistic way.

Instead of completely trusting the contour obtained at frame t−1, adecision is made of how to update the observation models by using theforward-backward algorithm. The “forward probability distribution” isdefined as follow:α_(φ)(s)=p(O ₁ ,O ₂ , . . . ,O _(φ) ,s _(φ) =s)which can be computed using recursion as follows:

$\begin{matrix}{{\alpha_{1}(s)} = {{p\left( {s_{1} = s} \right)}{p\left( {{O_{1}❘s_{1}} = s} \right)}}} \\{{\alpha_{\phi + 1}(s)} = {\left\lbrack {\sum\limits_{u}\;{{\alpha_{\phi}(u)}a_{u,s}}} \right\rbrack{p\left( {{O_{\phi + 1}❘s_{\phi + 1}} = s} \right)}}}\end{matrix}$

Similarly, the “backward probability distribution” is defined as:β_(φ)(s)=p(O _(φ+1) ,O _(φ+2) , . . . ,O _(M) ,s _(φ) =s)which can be computed using recursion as follows:

$\begin{matrix}{{\beta_{M}(s)} = 1} \\{{\beta_{\phi}(s)} = {\sum\limits_{u}\;{a_{s,u}{p\left( {{O_{\phi + 1}❘s_{\phi + 1}} = u} \right)}{\beta_{\phi + 1}(u)}}}}\end{matrix}$

After computing the forward and backward probability, we can compute theprobability of each state at line φ as follows:

${{P\left( {s_{\phi} = {s❘O}} \right)} = \frac{{\alpha_{\phi}(s)}{\beta_{\phi}(s)}}{\sum\limits_{u}\;{{\alpha_{\phi}(u)}{\beta_{\phi}(u)}}}},{s \in \left\lbrack {{- N},N} \right\rbrack}$which represents the probability of having the contour point at s on themeasurement line φ.

Based on these probabilities, the probability of pixel λ_(φ) being inthe foreground (or background) can be computed by integratingP(s_(φ)=s|O) along the normal line as follows:

${P\left( {\lambda_{\phi} \in {BG}} \right)} = {{1 - {P\left( {\lambda_{\phi} \in {FG}} \right)}} = {\prod\limits_{s = {- N}}^{\lambda_{\phi}}\;{p\left( {s_{\phi} = {s❘O}} \right)}}}$

This probability gives us a robust way to weigh different pixels duringadaptation of the observation models. The more confidently classifiedpixels contribute more to the color model while the less confidentlyclassified pixels contribute less:

$\begin{matrix}\begin{matrix}{{p\left( {v❘{BG}} \right)} = \frac{\sum\limits_{s = {- N}}^{N}\;{{P\left( {s \in {BG}} \right)} \cdot {O_{\phi}(s)}}}{\sum\limits_{s = {- N}}^{N}\;{P\left( {s \in {BG}} \right)}}} \\{{p\left( {v❘{FG}} \right)} = \frac{\sum\limits_{s = {- N}}^{N}\;{{P\left( {s \in {FG}} \right)} \cdot {O_{\phi}(s)}}}{\sum\limits_{s = {- N}}^{N}\;{P\left( {s \in {FG}} \right)}}}\end{matrix} & (6)\end{matrix}$

The new adapted models reflect the changing color distributions duringthe tracking. The new adapted models are then plugged back into Equation(1) during the contour searching in the next frame. In the illustratedexample, the transition probabilities are not trained because theytypically tend to remain relatively constant during the trackingprocess. Alternatively, the transition probabilities may be trained in amanner analogous to the training of the color distributions.

Returning to FIG. 9, the multi-cue tracking process 400 can be furtherseen with reference to FIG. 14. FIG. 14 illustrates the tracking of anobject from one frame 522 at time t−1 to the next frame 524 at time t. Aprediction is made (act 402) of where the object will be in the currentframe t based on the tracking results in previous frame t−1 and theobject's dynamics. Observations are collected along a set of normallines of the predicted contour (act 404). The well-known Langevinprocess is used to model the human movement dynamics:

$\begin{bmatrix}\theta_{t} \\{\overset{.}{\theta}}_{t}\end{bmatrix} = {{\begin{bmatrix}1 & \tau \\0 & a\end{bmatrix}\begin{bmatrix}\theta_{t - 1} \\{\overset{.}{\theta}}_{t - 1}\end{bmatrix}} + {\begin{bmatrix}0 \\b\end{bmatrix}m_{t}}}$where θ=[x, y, α,β,φ] is the parametric ellipse, a=exp(−β_(θ)τ), b=ν√{square root over (1−a²)}. β_(θ) is the rate constant, m is a thermalexcitation process drawn from Gaussian distribution N(0, Q), τ is thediscretization time step, and ν is the steady-state root-mean-squarevelocity.

The observation likelihood function is evaluated (act 406) for everypixel on normal line φ:p(O _(φ) |s _(φ)=λ_(φ)),λ_(φ) ∈[−N,N],φ∈[1,M]based on edge detection and the color value of each pixel on the line byusing calculation (2) above. The state transition probabilities based onJPDAF are also evaluated (act 408) as shown in calculation (4) above.

With the previously computed observation likelihood and the transitionprobability matrix, the best contour with respect to the givenobservations is found by the Viterbi Algorithm (act 410), and, based onthe detected contour, the best ellipse is fit (act 412) usingcalculation (6) above.

Then, using forward-backward algorithm to estimate a soft classificationof each pixel (to foreground and background) on the normal lines, updatethe color model of foreground and background based on calculation (6)above (act 414).

The process 400 of FIG. 9 is repeated for each frame of the videocontent.

Multi-cue tracking module 144 attempts to account for cue reliabilityand changes in cue reliability. For example, the properties of bothforeground and background are modeled (see, calculation (1) above), andthe model is used in calculation (2) above to detect the boundary (e.g.,if the color of the foreground and background are similar, it will notcontribute much for the boundary detection and the process will relymore on other cues that are more discriminant, such as motion). Themodel of the background and foreground is also adapted during thetracking which is expressed by calculation (6) above.

Various modifications can also be made to the multi-cue tracking processdiscussed above. According to one alternative, a set of one or morefeature points of the face being tracked is maintained and each newframe is analyzed to locate that set of feature points. Once the set offeature points is located, the position of the face can be estimated ata coarse level based on the located set of points, and then this coarseestimation used as the initial guess in the parametric contour-trackingprocess discussed above. In other words, the new frame is analyzed tolocate an initial guess for the parametric contour-tracking processrather than relying on the predicted location discussed above. Thismodification can be particularly useful in situations where the objectmotion between successive frames is large (large enough that thepredicted location discussed above may not be close enough to the actualcontour location in the subsequent frame).

A variety of different feature points can be tracked, such as eyecorners, mouth corners, nostrils, etc. Sound sources in the audio canalso be tracked as feature points, in addition to or in place of thevisual features. A variety of different feature tracking processes canbe used, such as the well-known Lucas-Kanade feature tracker. Additionalinformation regarding the Lucas-Kanade feature tracker can be found inJ. Shi and C. Tomasi, “Good Features to Track,” IEEE Conf. on ComputerVision and Pattern Recognition, pp. 593-600, 1994.

Another modification that can be made to the multi-cue tracking processdiscussed above is, when performing probabilistic sampling, to samplefrom the feature points (detected contour points) rather than from thestate space. For example, several contour points could be sampled fromall the detected contour points and a parametric shape fit on thesampled contour points.

Another modification that can be made to the multi-cue tracking processis to track multiple possible locations for the face—in other words,track multiple hypotheses rather than one hypothesis. Particle-filteringtechniques can be used to maintain multiple hypotheses so that weakhypotheses are not immediately dropped. Rather, weak hypotheses aremaintained and allowed time to prove they are good choices. Next isdescribed one of such particle filter techniques, referred to asunscented particle filter.

An unscented particle filter (UPF) that uses an unscented Kalman filter(UKF) is used by multi-cue tracking module 144 to track multiplehypotheses. The unscented transformation (UT) is used to compute themean and covariance up to the second order (third for Gaussian prior) ofthe Taylor series expansion of g( ). Let n_(x) be the dimension of x, xbe the mean of x, and P_(x) be the covariance of x, the UT computes meanand covariance of y=g(x) as follows:

First, deterministically generate 2n_(x)+1 sigma points S_(i)=(X_(i),W_(i)}:X₀= xX _(i) = x +(√{square root over ((n _(x)+λ)P _(x))})_(i) i=1, . . . ,n_(x)X _(i) = x −(√{square root over ((n _(x)+λ)P _(x))})_(i) i=n _(x)+1, . .. ,2n _(x)W ₀ ^((m))=λ/(n _(x)+λ), W₀ ^((c)) =W ₀ ^((m))+(1−α²+β)  (7)W _(i) ^((m)) =W _(i) ^((m))=1/(2·(n _(x)+λ)) i1, . . . ,2n _(x)λ=α²(n _(x)+κ)−n _(x)where κ is a scaling parameter that controls the distance between thesigma points and the mean x, α is a positive scaling parameter thatcontrols the higher order effects resulted from the non-linear functiong( ), and β is a parameter that controls the weighting of the 0^(th)sigma point, and (√{square root over ((n_(x)+λ)P_(x))})_(i) is thei^(th) column of the matrix square root. In one implementation, for thescalar case, α=1 , β=0 and κ=2. Note that the 0^(th) sigma point'sweight is different for calculating mean and covariance.

Then, the sigma points are propagated through the nonlineartransformation:Y _(i) =g(X _(i)) i=0, . . . 2n _(x)   (8)and the mean and covariance of y are calculated as follows:

$\begin{matrix}{{\overset{\_}{y} = {\sum\limits_{i = 0}^{2n_{x}}\;{W_{i}^{(m)}Y_{i}}}},{P_{y} = {\sum\limits_{i = 0}^{2n_{x}}\;{{W_{i}^{(c)}\left( {Y_{i} - \overset{\_}{y}} \right)}\left( {Y_{i} - \overset{\_}{y}} \right)^{T}}}}} & (9)\end{matrix}$The mean and covariance of y is accurate up to the second order of theTaylor series expansion.

The unscented Kalman filter (UKF) can be implemented using UT byexpanding the state space to include the noise component: x_(t)^(a)=[x_(t) ^(T)m_(t) ^(T)n_(t) ^(T)]^(T). Let N_(a)=N_(x)+N_(m)+N_(n)be the dimension of the expanded state space, where N_(m) and N_(n) arethe dimensions of noise m_(t) and n_(t), and Q and R be the covariancefor noise m_(t) and n_(t), the UKF can be summarized as follows:

Initialization:

$\begin{matrix}{{{\overset{\_}{x}}_{0}^{a} = \left\lbrack {{\overset{\_}{x}}_{0}^{T}00} \right\rbrack^{T}},{P_{0}^{a} = \begin{bmatrix}P_{0} & 0 & 0 \\0 & Q & 0 \\0 & 0 & R\end{bmatrix}}} & (10)\end{matrix}$

Iterate the following for each time instance t:

a) Calculate the sigma points using the procedure in calculation 7above:X _(t−1) ^(a) =[ x _(t−1) ^(a) x _(t−1) ^(a)±√{square root over ((n_(a)+λ)P _(t−1) ^(a))}]  (11)

b) Time update:

$\begin{matrix}{{X_{t❘{t - 1}}^{x} = {f\left( {X_{t - 1}^{x},X_{t - 1}^{v}} \right)}},{{\overset{\_}{x}}_{t❘{t - 1}} = {\sum\limits_{i = 0}^{2n_{a}}\;{W_{i}^{(m)}X_{i,{t❘{t - 1}}}^{x}}}}} & (12) \\{{Y_{t❘{t - 1}} = {h\left( {X_{t❘{t - 1}}^{x},X_{t - 1}^{n}} \right)}},{{\overset{\_}{y}}_{t❘{t - 1}} = {\sum\limits_{i = 0}^{2n_{a}}\;{W_{i}^{(m)}Y_{i,{t❘{t - 1}}}^{x}}}}} & (13) \\{P_{t❘{t - 1}} = {\sum\limits_{i = 0}^{2n_{a}}\;{{W_{i}^{(c)}\left\lbrack {X_{i,{t❘{t - 1}}}^{x} - {\overset{\_}{x}}_{t❘{t - 1}}} \right\rbrack}\left\lbrack {X_{i,{t❘{t - 1}}}^{x} - {\overset{\_}{x}}_{t❘{t - 1}}} \right\rbrack}^{T}}} & (14)\end{matrix}$

c) Measurement update:

$\begin{matrix}{P_{y_{t}y_{t}} = {\sum\limits_{i = 0}^{2n_{a}}\;{{W_{i}^{(c)}\left\lbrack {Y_{i,{t❘{t - 1}}} - {\overset{\_}{y}}_{t❘{t - 1}}} \right\rbrack}\left\lbrack {Y_{i,{t❘{t - 1}}} - {\overset{\_}{y}}_{t❘{t - 1}}} \right\rbrack}^{T}}} & (15) \\{P_{x_{t}y_{t}} = {\sum\limits_{i = 0}^{2n_{a}}\;{{W_{i}^{(c)}\left\lbrack {X_{i,{t❘{t - 1}}}^{x} - {\overset{\_}{x}}_{t❘{t - 1}}} \right\rbrack}\left\lbrack {Y_{i,{t❘{t - 1}}}^{x} - {\overset{\_}{y}}_{t❘{t - 1}}} \right\rbrack}^{T}}} & (16) \\{K_{t} = {P_{x_{t}y_{t}}P_{y_{t}y_{t}}^{- 1}}} & (17) \\{{{\overset{\_}{x}}_{t} = {{\overset{\_}{x}}_{t❘{t - 1}} + {K_{t}\left( {y_{t} - {\overset{\_}{y}}_{t❘{t - 1}}} \right)}}},\;{P_{t} = {P_{t❘{t - 1}} - {K_{t}P_{y_{t}y_{t}}K_{t}^{T}}}}} & (18)\end{matrix}$

With UKF, the most recent observation can be easily incorporated intothe state estimation (e.g., measure update c) above); however, it makesa Gaussian assumption of the state distribution. The particle filters,on the other hand, can model arbitrary distributions, but incorporatingnew observation y_(t) into the proposal distribution is difficult. UKFis used to generate the proposal distribution for the particle filter,resulting in the hybrid UPF. Specifically, the proposal distribution foreach particle is as follows:q(x _(t) ^((i)) |x _(0:t−1) ^((i)) y _(1:t))=N( x _(t) ^((i)) ,P _(t)^((i))), i=1, . . . ,N   (19)where x _(t) and P_(t) are the mean and covariance of x, computed usingUKF (calculations (10)-(18)). It should be noted that, even though theGaussian assumption is not realistic to approximate the posteriordistribution p(x_(t)|x_(t−1),y_(0:t)), it is less a problem to generateindividual particles with distinct x _(t) and P_(t). Furthermore,because UKF approximates the mean and covariance of the posterior up tothe second order, the non-linearity of system is well preserved. The UPFprocess is easily obtained by plugging the UKF step and calculation (19)into the generic particle filter algorithm.

FIG. 15 is a flowchart illustrating an exemplary UPF process 550. Theprocess of FIG. 15 is performed by multi-cue tracking module 144 of FIG.3, and may be performed in software.

Initially, particles x_(t) ^((i)), i=1, . . . , N, are updated with theUKF using calculations (11)-(18) to obtain x _(t) ^((i)) and p_(t)^((i)) (act 552). Particles x_(t) ^((i)), i=1, . . . , N, are thensampled from the proposal distribution q(x_(t) ^((i))|x_(0:t−1) ^((i)),y_(1:t))=N ( x _(t) ^((i)), P_(t) ^((i))) (act 554). The particleweights are then computed (act 556) using calculation (20) as follows:

$\begin{matrix}\begin{matrix}{{\overset{\sim}{w}}_{t}^{(i)} = \frac{{p\left( {y_{1\text{:}t}❘x_{0\text{:}t}^{(i)}} \right)}{p\left( x_{0\text{:}t}^{(i)} \right)}}{{q\left( {x_{{0\text{:}t} - 1}^{(i)}❘y_{{1\text{:}t} - 1}} \right)}{q\left( {{x_{t}^{(i)}❘x_{{0\text{:}t} - 1}^{(i)}},y_{1\text{:}t}} \right)}}} \\{= {{\overset{\sim}{w}}_{t - 1}^{(i)}\frac{{p\left( {y_{1\text{:}t}❘x_{0\text{:}t}^{(i)}} \right)}{p\left( x_{0\text{:}t}^{(i)} \right)}}{{p\left( {y_{{1\text{:}t} - 1}❘x_{{0\text{:}t} - 1}^{(i)}} \right)}{p\left( x_{{0\text{:}t} - 1}^{(i)} \right)}{q\left( {{x_{t}^{(i)}❘x_{{0\text{:}t} - 1}^{(i)}},y_{1\text{:}t}} \right)}}}} \\{= {{\overset{\sim}{w}}_{t - 1}^{(i)}\frac{{p\left( {y_{t}❘x_{t}^{(i)}} \right)}{p\left( {x_{t}^{(i)}❘x_{t - 1}^{(i)}} \right)}}{q\left( {{x_{t}^{(i)}❘x_{{0\text{:}t} - 1}^{(i)}},y_{1\text{:}t}} \right)}}}\end{matrix} & (20)\end{matrix}$

The importance weight is then normalized (act 558) using calculation(21) as follows:

$\begin{matrix}{{w_{t}\left( x_{0\text{:}t}^{(i)} \right)} = {{{\overset{\sim}{w}}_{t}\left( x_{0\text{:}t}^{(i)} \right)}/{\sum\limits_{i = 1}^{N}\;{{\overset{\sim}{w}}_{t}\left( x_{0\text{:}t}^{(i)} \right)}}}} & (21)\end{matrix}$where the particles {x_(0:t) ^((i)), w_(t)(x_(0:t) ^((i)))} are drawnfrom the known distribution q, {tilde over (w)}_(t)(x_(0:t) ^((i))) andw_(t)(x_(0:t) ^((i))) are the un-normalized and normalized importanceweights.

The effective particle size S is then determined (act 560) usingcalculation (22) as follows:

$\begin{matrix}{{\overset{\sim}{w}}_{t}^{(i)} = {{{\overset{\sim}{w}}_{t - 1}^{(i)}\frac{{p\left( {y_{t}❘x_{t}^{(i)}} \right)}{p\left( {x_{t}^{(i)}❘x_{t - 1}^{(i)}} \right)}}{q\left( {{x_{t}^{(i)}❘x_{{0\text{:}t} - 1}^{(i)}},y_{1\text{:}t}} \right)}} = {{\overset{\sim}{w}}_{t - 1}^{(i)}{p\left( {y_{t}❘x_{t}^{(i)}} \right)}}}} & (22)\end{matrix}$If S<S_(T), then multiply (or suppress) weighted particles to generate Nequal-weighted particles (act 562). The expectations of g( ) are thencomputed (act 564) using calculation (23) as follows:

$\begin{matrix}{{E_{p}\left( {g\left( x_{0\text{:}t} \right)} \right)} = {\lim_{N->\infty}{\sum\limits_{i = 1}^{N}\;{{q\left( x_{0\text{:}t}^{(i)} \right)}{w_{t}\left( x_{0\text{:}t}^{(i)} \right)}}}}} & (23)\end{matrix}$The conditional mean of x_(t) can be computed with g_(t)(x_(t))=x_(t),and conditional covariance of x_(t) can be computed withg_(t)(x_(t))=x_(t)x_(t) ^(T).

Using the UPF process 550 of FIG. 15 to track participants based onaudio will now be discussed. Two microphones are typically sufficient toestimate the horizontal panning angle. Tracking based on the horizontalpanning angle is discussed herein, and analogous operations can beperformed to track based on the vertical tilting angle of the speaker.FIG. 16 illustrates an exemplary multiple-microphone environment. InFIG. 16, assume the two microphones are situated at locations A and B,and the sound source is situated at location C. When the distance of thesound source (i.e., |OC|) is much larger than the length of themicrophone pair baseline |AB|, the panning angle θ=∠COX can be estimatedas follows:

$\begin{matrix}{\theta = {{{\angle\;{COX}} \approx {\angle\;{BAE}}} = {{\arcsin\frac{{BE}}{{AB}}} = {\arcsin\frac{D \times v}{{AB}}}}}} & (24)\end{matrix}$where D is the time delay between the two microphones, and ν=342 m/s isthe speed of sound traveling in air.

In order to utilize the UPF framework in a tracking application, fourentities are first established: system dynamics x_(t)=f(x_(t−1),m_(t−1))to be used in calculation (12), system observation y_(t)=h(x_(t),n_(t))to be used in calculation (13), likelihood p(y_(t)|x_(t)) to be used incalculation (22), and innovation y_(t)− y _(t|t−1) to be used incalculation (18). Once these four entities are established, trackingproceeds straightforwardly using the UPF process 550 of FIG. 15.

The system dynamics model x_(t)=f(x_(t−1),m_(t−1)) is determined asfollows. Let x=[θ,{dot over (θ)}]^(T) be the state space, where they arethe panning angle and velocity of the panning angle, respectively. Tomodel the movement dynamics of a talking person, the well-known Langevinprocess d²θ/dt²+β₀·dθ/dt=m is used, whose discrete form is:

$\begin{matrix}{{\begin{bmatrix}\theta_{t} \\{\overset{.}{\theta}}_{t}\end{bmatrix} = {{\begin{bmatrix}1 & \tau \\0 & a\end{bmatrix}\begin{bmatrix}\theta_{t - 1} \\{\overset{.}{\theta}}_{t - 1}\end{bmatrix}} + {\begin{bmatrix}0 \\b\end{bmatrix}m_{t}}}}{{a = {\exp\left( {{- \beta_{\theta}}\tau} \right)}},{b = {\overset{\_}{v}\sqrt{1 - a^{2}}}}}} & (25)\end{matrix}$where β_(θ) is the rate constant, m is a thermal excitation processdrawn from N(0,Q), τ is the discretization time step, and ν is thesteady-state root−mean-square velocity.

The system observation model y_(t)=h(x_(t),n_(t)) is determined asfollows. The system observation y_(t) is the time delay D_(t). Based oncalculation (24) above, the observation relates to the state byy _(t) =D _(t) =h(θ_(t) ,n _(t))=|AB|ν sin θ_(t) +n _(t)   (26)where n_(t) is the measurement noise, obeying a Gaussian distribution ofN(0, R).

The likelihood model p(y_(t)|x_(t)) is determined as follows. Let J bethe number of peaks in the GCCF (generalized cross-correlationfunction). Of the J peak locations, at most one is from the true soundsource. Therefore, define J+1 hypotheses can be defined:H ₀ ={c _(j) =C:j=1, . . . , J}H _(j) ={c _(j) =T,c _(k) =C:k=1, . . . ,J,k≠j }  (27)where c_(j)=T means the j^(th) peak is associated with the true soundsource, c_(j)=C otherwise. Hypothesis H₀ therefore means that none ofthe peaks is associated with the true source. The combined likelihoodmodel is therefore:

$\begin{matrix}{\begin{matrix}{{p\left( {y_{t}❘x_{t}} \right)} = {{\pi_{0}{p\left( {y_{t}❘H_{0}} \right)}} + {\sum\limits_{j = 1}^{J}\;{\pi_{j}{p\left( {y_{t}❘H_{j}} \right)}}}}} \\{= {{\pi_{0}U} + {N_{m}{\sum\limits_{j = 1}^{J}\;{\pi_{j}{N\left( {D_{j},\sigma_{D}} \right)}}}}}}\end{matrix}{{{s.t.\mspace{14mu}\pi_{0}} + {\sum\limits_{j = 1}^{J}\;\pi_{j}}} = 1}} & (28)\end{matrix}$where π₀ is the prior probability of hypothesis H₀, π_(j),j=1, . . . ,J, can be obtained from the relative height of the j^(th) peak, N_(m) isa normalization factor, D_(j) is the time delay corresponding the j^(th)peak, U represents the uniform distribution, and N( ) represents theGaussian distribution.

The innovation model y_(t)− y _(t|t−1) is determined as follows. Thesame as the likelihood model, the innovation model also needs to takeinto account the multi-peak fact:

$\begin{matrix}{{y_{t} - {\overset{\_}{y}}_{t❘{t - 1}}} = {\sum\limits_{j = 1}^{J}\;{\pi_{j}\left( {D_{j} - {\overset{\_}{y}}_{t❘{t - 1}}} \right)}}} & (29)\end{matrix}$where y _(t|t−1) is the predicted measurement obtained from UKF (seecalculation (18) above).

Using the UPF process 550 of FIG. 15 to track participants based onvisual data is similar to that of tracking participants based on audibledata. In order to utilize the UPF framework in a tracking application,four entities are first established: the system dynamics modelx_(t)=f(x_(t−1),m_(t−1)), the system observation modely_(t)=h(x_(t),n_(t)), the likelihood model p(y_(t)|x_(t)) and theinnovation model y_(t)− y _(t|t−1). Once these four entities areestablished, tracking proceeds straightforwardly using the UPF process550 of FIG. 15.

The system dynamics model x_(t)=f(x_(t−1),m_(t−1)) is determined asfollows. Let (r, s) represent the image coordinate. In contour-basedtracking, the system states are the position of the ellipse center andits horizontal and vertical velocity, i.e., x_(t)=[r_(t),s_(t),{dot over(r)}_(t),{dot over (s)}_(t)]^(T). Similar to the system dynamics modelfor audible data, the well-known Langevin process is adopted to modelthe human movement dynamics:

$\begin{matrix}{\begin{bmatrix}r_{t} \\s_{t} \\{\overset{.}{r}}_{t} \\{\overset{.}{s}}_{t}\end{bmatrix} = {{\begin{bmatrix}1 & 0 & \tau & 0 \\0 & 1 & 0 & \tau \\0 & 0 & a_{r} & 0 \\0 & 0 & 0 & a_{\overset{.}{s}}\end{bmatrix}\begin{bmatrix}r_{t - 1} \\s_{t - 1} \\{\overset{.}{r}}_{t - 1} \\{\overset{.}{s}}_{t - 1}\end{bmatrix}} + {\begin{bmatrix}0 \\0 \\b_{r} \\b_{s}\end{bmatrix}m_{t}}}} & (30)\end{matrix}$

The system observation model y_(t)=h(x_(t),n_(t)) is determined asfollows. The ellipse is centered at the current state location(r_(t),s_(t)). K rays are generated from the ellipse center andintersect with the ellipse boundary. The ellipse center is used as theorigin of a local coordinate system, so the intersections (u_(k),v_(k)), k=1, 2, . . . , K, can be obtained as

$\begin{matrix}{{u_{k} = \sqrt{\tan^{2}{\varphi_{k}/\left( {{1.44\mspace{14mu}\tan^{2}\varphi_{k}} + 1} \right)}}}{v_{k} = \sqrt{1/\left( {{1.44\mspace{14mu}\tan^{2}\varphi_{k}} + 1} \right)}}} & (31)\end{matrix}$by jointly solving the ellipse equation and the ray equation:

$\begin{matrix}\left\{ \begin{matrix}{{\frac{u_{k}^{2}}{1} + \frac{v_{k}^{2}}{1.2^{2}}} = 1} \\{u_{k} = {v_{k}{\tan\left( \varphi_{k} \right)}}}\end{matrix} \right. & (32)\end{matrix}$

Transforming the local (u, v) coordinate back to the image coordinate,the following observation is obtained:

$\begin{matrix}\begin{matrix}{y_{t} = {h\left( {x_{t},n_{t}} \right)}} \\{{= {\left\lbrack \left( {{u_{k} + r_{t}},{v_{k} + s_{t}}} \right) \right\rbrack + n_{t}}},{k = 1},2,\ldots\mspace{11mu},{K.}}\end{matrix} & (33)\end{matrix}$where n_(t) is the measurement noise, obeying a Gaussian distribution ofN(0, R). It should be noted that the observation model is highlynon-linear.

The likelihood model p(y_(t)|x_(t)) is determined as follows. The edgeintensity is used to model the state likelihood. Along each of the Krays, the well-known Canny edge detector is used to calculate the edgeintensity. The resulting function is a multi-peak function, just likethe GCCF in the likelihood model for audible data. The multiple peakssignify there are multiple edge candidates along this ray. Let thenumber of peaks be J, we can use the same likelihood model developed inthe likelihood model for audible data to model the edge likelihood alongray k:

$\begin{matrix}{{p^{(k)}\left( {y_{t}❘x_{t}} \right)} = {{\pi_{k0}{p^{(k)}\left( {y_{t}❘H_{0}} \right)}} + {\sum\limits_{j = 1}^{J}{\pi_{kj}{p^{(k)}\left( {y_{t}❘H_{j}} \right)}}}}} \\{= {{\pi_{k0}U} + {N_{m}{\sum\limits_{j = 1}^{J}{\pi_{kj}{N\left( {\left( {u_{k},v_{k}} \right)_{j},\sigma_{kj}} \right)}}}}}}\end{matrix}$

The overall likelihood considering all the K rays is therefore:

$\begin{matrix}{{p\left( {y_{t}❘x_{t}} \right)} = {\prod\limits_{k = 1}^{K}\;{p^{(k)}\left( {y_{t}❘x_{t}} \right)}}} & (34)\end{matrix}$

The innovation model y_(t)− y _(t|t−1) is determined as follows. Thesame as the likelihood model, the innovation model also needs to takeinto account the multi-peak fact:

${y_{t}^{(k)} - {\overset{\_}{y}}_{t❘{t - 1}}^{(k)}} = {\sum\limits_{j = 1}^{J}{\pi_{kj}\left( {\left( {u_{k},v_{k}} \right)_{t,j} - \left( {u_{k},v_{k}} \right)_{t❘{t - 1}}} \right)}}$where k=1, 2, . . . , K, π_(kj) is the mixing weight for the j^(th) peakalong ray k, and can be obtained from the corresponding edge intensity.General Computer Environment

FIG. 17 illustrates a general computer environment 600, which can beused to implement the automatic detection and tracking of multipleindividuals described herein. The computer environment 600 is only oneexample of a computing environment and is not intended to suggest anylimitation as to the scope of use or functionality of the computer andnetwork architectures. Neither should the computer environment 600 beinterpreted as having any dependency or requirement relating to any oneor combination of components illustrated in the exemplary computerenvironment 600.

Computer environment 600 includes a general-purpose computing device inthe form of a computer 602. Computer 602 can be, for example, a system102 of FIG. 1, a system 112 of FIG. 2, a system 130 of FIG. 3, etc. Thecomponents of computer 602 can include, but are not limited to, one ormore processors or processing units 604, a system memory 606, and asystem bus 608 that couples various system components including theprocessor 604 to the system memory 606.

The system bus 608 represents one or more of any of several types of busstructures, including a memory bus or memory controller, a peripheralbus, an accelerated graphics port, and a processor or local bus usingany of a variety of bus architectures. By way of example, sucharchitectures can include an Industry Standard Architecture (ISA) bus, aMicro Channel Architecture (MCA) bus, an Enhanced ISA (EISA) bus, aVideo Electronics Standards Association (VESA) local bus, and aPeripheral Component Interconnects (PCI) bus also known as a Mezzaninebus.

Computer 602 typically includes a variety of computer readable media.Such media can be any available media that is accessible by computer 602and includes both volatile and non-volatile media, removable andnon-removable media.

The system memory 606 includes computer readable media in the form ofvolatile memory, such as random access memory (RAM) 610, and/ornon-volatile memory, such as read only memory (ROM) 612. A basicinput/output system (BIOS) 614, containing the basic routines that helpto transfer information between elements within computer 602, such asduring start-up, is stored in ROM 612. RAM 610 typically contains dataand/or program modules that are immediately accessible to and/orpresently operated on by the processing unit 604.

Computer 602 may also include other removable/non-removable,volatile/non-volatile computer storage media. By way of example, FIG. 17illustrates a hard disk drive 616 for reading from and writing to anon-removable, non-volatile magnetic media (not shown), a magnetic diskdrive 618 for reading from and writing to a removable, non-volatilemagnetic disk 620 (e.g., a “floppy disk”), and an optical disk drive 622for reading from and/or writing to a removable, non-volatile opticaldisk 624 such as a CD-ROM, DVD-ROM, or other optical media. The harddisk drive 616, magnetic disk drive 618, and optical disk drive 622 areeach connected to the system bus 608 by one or more data mediainterfaces 626. Alternatively, the hard disk drive 616, magnetic diskdrive 618, and optical disk drive 622 can be connected to the system bus608 by one or more interfaces (not shown).

The disk drives and their associated computer-readable media providenon-volatile storage of computer readable instructions, data structures,program modules, and other data for computer 602. Although the exampleillustrates a hard disk 616, a removable magnetic disk 620, and aremovable optical disk 624, it is to be appreciated that other types ofcomputer readable media which can store data that is accessible by acomputer, such as magnetic cassettes or other magnetic storage devices,flash memory cards, CD-ROM, digital versatile disks (DVD) or otheroptical storage, random access memories (RAM), read only memories (ROM),electrically erasable programmable read-only memory (EEPROM), and thelike, can also be utilized to implement the exemplary computing systemand environment.

Any number of program modules can be stored on the hard disk 616,magnetic disk 620, optical disk 624, ROM 612, and/or RAM 610, includingby way of example, an operating system 626, one or more applicationprograms 628, other program modules 630, and program data 632. Each ofsuch operating system 626, one or more application programs 628, otherprogram modules 630, and program data 632 (or some combination thereof)may implement all or part of the resident components that support thedistributed file system.

A user can enter commands and information into computer 602 via inputdevices such as a keyboard 634 and a pointing device 636 (e.g., a“mouse”). Other input devices 638 (not shown specifically) may include amicrophone, joystick, game pad, satellite dish, serial port, scanner,and/or the like. These and other input devices are connected to theprocessing unit 604 via input/output interfaces 640 that are coupled tothe system bus 608, but may be connected by other interface and busstructures, such as a parallel port, game port, or a universal serialbus (USB).

A monitor 642 or other type of display device can also be connected tothe system bus 608 via an interface, such as a video adapter 644. Inaddition to the monitor 642, other output peripheral devices can includecomponents such as speakers (not shown) and a printer 646 which can beconnected to computer 602 via the input/output interfaces 640.

Computer 602 can operate in a networked environment using logicalconnections to one or more remote computers, such as a remote computingdevice 648. By way of example, the remote computing device 648 can be apersonal computer, portable computer, a server, a router, a networkcomputer, a peer device or other common network node, and the like. Theremote computing device 648 is illustrated as a portable computer thatcan include many or all of the elements and features described hereinrelative to computer 602.

Logical connections between computer 602 and the remote computer 648 aredepicted as a local area network (LAN) 650 and a general wide areanetwork (WAN) 652. Such networking environments are commonplace inoffices, enterprise-wide computer networks, intranets, and the Internet.

When implemented in a LAN networking environment, the computer 602 isconnected to a local network 650 via a network interface or adapter 654.When implemented in a WAN networking environment, the computer 602typically includes a modem 656 or other means for establishingcommunications over the wide network 652. The modem 656, which can beinternal or external to computer 602, can be connected to the system bus608 via the input/output interfaces 640 or other appropriate mechanisms.It is to be appreciated that the illustrated network connections areexemplary and that other means of establishing communication link(s)between the computers 602 and 648 can be employed.

In a networked environment, such as that illustrated with computingenvironment 600, program modules depicted relative to the computer 602,or portions thereof, may be stored in a remote memory storage device. Byway of example, remote application programs 658 reside on a memorydevice of remote computer 648. For purposes of illustration, applicationprograms and other executable program components such as the operatingsystem are illustrated herein as discrete blocks, although it isrecognized that such programs and components reside at various times indifferent storage components of the computing device 602, and areexecuted by the data processor(s) of the computer.

An implementation of the distributed file system 150 may be described inthe general context of computer-executable instructions, such as programmodules, executed by one or more computers or other devices. Generally,program modules include routines, programs, objects, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. Typically, the functionality of the program modulesmay be combined or distributed as desired in various embodiments.

An implementation of the file format for the encrypted files may bestored on or transmitted across some form of computer readable media.Computer readable media can be any available media that can be accessedby a computer. By way of example, and not limitation, computer readablemedia may comprise “computer storage media” and “communications media.”

“Computer storage media” includes volatile and non-volatile, removableand non-removable media implemented in any method or technology forstorage of information such as computer readable instructions, datastructures, program modules, or other data. Computer storage mediaincludes, but is not limited to, RAM, ROM, EEPROM, flash memory or othermemory technology, CD-ROM, digital versatile disks (DVD) or otheroptical storage, magnetic cassettes, magnetic tape, magnetic diskstorage or other magnetic storage devices, or any other medium which canbe used to store the desired information and which can be accessed by acomputer.

“Communication media” typically embodies computer readable instructions,data structures, program modules, or other data in a modulated datasignal, such as carrier wave or other transport mechanism. Communicationmedia also includes any information delivery media. The term “modulateddata signal” means a signal that has one or more of its characteristicsset or changed in such a manner as to encode information in the signal.By way of example, and not limitation, communication media includeswired media such as a wired network or direct-wired connection, andwireless media such as acoustic, RF, infrared, and other wireless media.Combinations of any of the above are also included within the scope ofcomputer readable media.

Although discussed herein primarily with reference to human faces, otherobjects can be automatically detected and/or tracked analogous to thehuman faces discussed herein.

CONCLUSION

Although the description above uses language that is specific tostructural features and/or methodological acts, it is to be understoodthat the invention defined in the appended claims is not limited to thespecific features or acts described. Rather, the specific features andacts are disclosed as exemplary forms of implementing the invention.

1. One or more computer readable media having stored thereon a pluralityof instructions to detect a candidate region for an untracked face in aframe of content, wherein the plurality of instructions, when executedby one or more processors, causes the one or more processors to: detectwhether there is motion in the frame; if there is motion in the frame,then perform motion-based initialization to identify the candidateregion; detect whether there is audio in the frame; if there is audio inthe frame, then perform audio-based initialization to identify thecandidate region; and if there is neither motion in the frame nor audioin the frame, then use a fast face detector to identify the candidateregion; and wherein the plurality of instructions to performmotion-based initialization comprises instructions that cause the one ormore processors to: determine whether there is motion at a plurality ofpixels on a plurality of lines across the frame; generate a sum of framedifferences for a plurality of segments of multiple ones of theplurality of lines; select, for each of the multiple lines, the segmenthaving the largest sum; identify a smoothest region of the selectedsegments; check whether the smoothest region resembles a human upperbody; and extract, as the candidate area, the portion of the smoothestregion that resembles a human head.
 2. One or more computer readablemedia as recited in claim 1, wherein the instructions to determinewhether there is motion comprise instructions that cause the one or moreprocessors to: determine, for each of the plurality of pixels, whether adifference between an intensity value of the pixel in the frame and anintensity value of a corresponding pixel in one or more other framesexceeds a threshold value.
 3. A method to detect a candidate region foran untracked face in a frame of content, the method comprising:detecting whether there is motion in the frame; if there is motion inthe frame, then performing motion-based initialization to identify thecandidate region; detecting whether there is audio in the frame; ifthere is audio in the frame, then performing audio-based initializationto identify the candidate region; and if there is neither motion in theframe nor audio in the frame, then using a fast face detector toidentify the candidate region; and wherein the performing of themotion-based initialization comprises: determining whether there ismotion at a plurality of pixels on a plurality of lines across theframe; generating a sum of frame differences for a plurality of segmentsof multiple ones of the plurality of lines; selecting, for each of themultiple lines, the segment having the largest sum; identifying asmoothest region of the selected segments; checking whether thesmoothest region resembles a human upper body; and extracting, as thecandidate area, the portion of the smoothest region that resembles ahuman head.
 4. A method as recited in claim 3, wherein the determiningwhether there is motion comprises: determining, for each of theplurality of pixels, whether a difference between an intensity value ofthe pixel in the frame and an intensity value of a corresponding pixelin one or more other frames exceeds a threshold value.
 5. A systemcomprising: one or more processors; and one or more computer readablemedia having stored thereon a plurality of instructions to detect acandidate region for an untracked face in a frame of content, whereinthe plurality of instructions, when executed by the one or moreprocessors, causes the one or more processors to: detect whether thereis motion in the frame; if there is motion in the frame, then performmotion-based initialization to identify the candidate region; detectwhether there is audio in the frame; if there is audio in the frame,then perform audio-based initialization to identify the candidateregion; and if there is neither motion in the frame nor audio in theframe, then use a fast face detector to identify the candidate region;and wherein the plurality of instructions to perform motion-basedinitialization comprises instructions that cause the one or moreprocessors to: determine whether there is motion at a plurality ofpixels on a plurality of lines across the frame; generate a sum of framedifferences for a plurality of segments of multiple ones of theplurality of lines; select, for each of the multiple lines, the segmenthaving the largest sum; identify a smoothest region of the selectedsegments; check whether the smoothest region resembles a human upperbody; and extract, as the candidate area, the portion of the smoothestregion that resembles a human head.
 6. A system as recited in claim 5,wherein the instructions to determine whether there is motion compriseinstructions that cause the one or more processors to: determine, foreach of the plurality of pixels, whether a difference between anintensity value of the pixel in the frame and an intensity value of acorresponding pixel in one or more other frames exceeds a thresholdvalue.